In the semiconductor industry, devices are fabricated by a number of processes to produce precisely-defined structures of an ever-decreasing size. Even the slightest structural defect can ruin a semiconductor device, and so to avoid losses of time and effort, detection of defects is critical before a defective device is mass-produced or further processes are performed on a defective wafer. Fast, on-line detection of wafer defects is possible through the use of optical wafer inspection systems. For example, in one type of system, a two-dimensional image of a selected field of view of a wafer is obtained, and that field of view is compared to another view which, under ideal conditions, should be identical. The comparison of like fields of view can thus reveal irregularities which could indicate a defect.
Generally speaking, a semiconductor wafer may include a number of repetitive patterns, and any defects in the semiconductor wafer will generally produce a variance in those patterns. To streamline the inspection process, a mask or masks may be configured to block portions of the light corresponding to the repetitive aspects of the wafer. In the resulting comparison of fields of view, the signal corresponding to a defect or other non-blocked feature will thus stand out more readily from the background. Such blocking is generally referred to as “Fourier filtering.” See, for example, U.S. Pat. No. 5,970,168, issued to Montesanto et al, for a discussion of one type of Fourier filter.
However, prior Fourier filtering methodologies may be less than ideal when utilized in the field. For instance, a high-throughput wafer production facility will have a correspondingly high throughput need for wafer inspections. Fourier filtering in such an environment will require fast, accurate production of appropriate filters for different expected patterns. A reconfigurable filter is one way to meet the high demand for filters in such an environment, but such filters can introduce an additional level of complexity (and thus point of failure) into an already-complex environment. Reconfigurable Fourier filters based on MEMS and LCD technologies have been proposed for use in optical inspection tools, but such filters may not always be practical for certain applications. For instance, both LCD filters and micromirror arrays may have fill factors, extinction ratios, and transmittance characteristics that are less than ideal. Also, light may leak from “closed” micro-elements in a MEMS-based device. Additionally, resolution of reconfigurable filters may be limited by the size of the reconfigurable elements and the size of the filter, such as the number of reconfigurable elements in a row.
Fixed Fourier filters can have higher transmittance, higher resolution, and avoid problems with fill factors. Additionally, fixed filters are generally less physically complex than reconfigurable filters, but require labor and skill to construct. Furthermore, great care must be taken during the construction and use of any filter to avoid the introduction of contaminants to the inspection environment, and to precisely position blocking elements. In the case of a fixed filter, manual construction is not only time-consuming, but generally must be performed outside the inspection cleanroom in order to avoid contaminating the wafer or optical inspection tool.
There remains a need for a Fourier filtering system which can be used for inspecting a variety of wafers with minimal impact on inspection throughput and labor costs.